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This paper presents a comprehensive review of shear strength measurements in wetland soils, which can be used to make inferences of the influence of nutrients and sediments on wetland health. Ecosystem restoration is increasing across the Gulf of Mexico and in other coastal systems, with management questions related to soil strength among the most critical to address for the sustainability of restoration programs. An overview of geotechnical engineering principles is provided as a starting point to understand basic soil mechanics concepts of stress, effective stress, pore-water pressure, unit weight, and shear strength. The review of wetland shear strength measurements focuses on the hand-held vane shear, torvane, cone penetrometer, and wetland soil strength tester. This synthesis shows that vane shear measurements can identify the shear strength trend in horizontal and vertical spaces and may be an indicator of wetland soil strength. However, the significant uncertainty of the vane shear measurements may preclude making conclusions about shear strength values without further testing and calibration of the devices. The torvane results show considerable scatter such that it is not recommended for quantitative shear strength measurements. The cone penetrometer represents a technique that is independent of operators and provides a high density of measurements with depth. It signifies the state-of-practice of wetland shear strength testing and is a reasonable tool to measure spatial and temporal variations in soil strength and other geotechnical properties (e.g., pore-water pressure, soil moisture, resistivity, and temperature) in wetlands. The wetland soil strength tester provides insight into the wetland soil resistance in the first 15 cm, which is the zone where most belowground biomass is present. Recommended future research includes evaluating the uncertainty in all in-situ soil strength testing methods, developing relationships between different field instruments, and establishing consistent statistical methods and field-testing procedures to make inferences and assessments.

The mineralization of soil organic nitrogen in two floating marshes types in Louisiana, USA with different vegetation and mat thickness (and biomass) was measured by incubating peat soil in situ in...

Wetland plants are subject to a range of physical stresses (e.g. inundation, salinity) that affect their productivity or health, which in turn may translate into wetland soils that vary in resistance to physical perturbations in the coastal setting. A primary goal of this study was to test a newly developed instrument designed to measure in-situ resistance to shear failure (soil strength) of marsh soils. The Wetland Soil Strength Tester (WSST) was used at 11 marsh types in coastal Louisiana, where soil bulk density ranged from organic to mineral (0.02–1.24 g cm−3). Based on analyses of live and dead components of both above and belowground biomass, live belowground biomass explained the most variation in marsh soil strength among the vegetation types. The WSST was capable of detecting in-situ live root biomass differences for 8 of 11 marsh types, where only the young deltaic marsh types were not significant. For all the sample plots (n = 227), an increase of 10-Nm soil strength corresponded to an increase of 200 g m−2 of live belowground biomass (R2 = 0.35, p

Deltas are globally important locations of diverse ecosystems, human settlement, and economic activity that are threatened by reductions in sediment delivery, accelerated sea level rise, and subsidence. Here we investigated the relative contribution of river flooding, hurricanes, and cold fronts on elevation change in the prograding Wax Lake Delta (WLD). Sediment surface elevation was measured across 87 plots, eight times from February 2008 to August 2011. The high peak discharge river floods in 2008 and 2011 resulted in the greatest mean net elevation gain of 5.4 to 4.9 cm over each flood season, respectively. The highest deltaic wetland sediment retention (13.5% of total sediment discharge) occurred during the 2008 river flood despite lower total and peak discharge compared to 2011. Hurricanes Gustav and Ike resulted in a total net elevation gain of 1.2 cm, but the long-term contribution of hurricane derived sediments to deltaic wetlands was estimated to be just 22% of the long-term contribution of large river floods. Winter cold front passage resulted in a net loss in elevation that is equal to the elevation gain from lower discharge river floods and was consistent across years. This amount of annual loss in elevation from cold fronts could effectively negate the long-term land building capacity within the delta without the added elevation gain from both high and low discharge river floods. The current lack of inclusion of cold front elevation loss in most predictive numerical models likely overestimates the land building capacity in areas that experience similar forcings to WLD.

Coastal marshes take up atmospheric CO2 while emitting CO2, CH4, and N2O. This ability to sequester carbon (C) is much greater for wetlands on a per area basis than from most ecosystems, facilitating scientific, political, and economic interest in their value as greenhouse gas sinks. However, the greenhouse gas balance of Gulf of Mexico wetlands is particularly understudied. We describe the net ecosystem exchange (NEEc) of CO2 and CH4 using eddy covariance (EC) in comparison with fluxes of CO2, CH4, and N2O using chambers from brackish and freshwater marshes in Louisiana, USA. From EC, we found that 182 g C m−2 yr−1 was lost through NEEc from the brackish marsh. Of this, 11 g C m−2 yr−1 resulted from net CH4 emissions and the remaining 171 g C m−2 yr−1 resulted from net CO2 emissions. In contrast, −290 g C m2 yr−1 was taken up through NEEc by the freshwater marsh, with 47 g C m−2 yr−1 emitted as CH4 and −337 g C m−2 yr−1 taken up as CO2. From chambers, we discovered that neither site had large fluxes of N2O. Sustained-flux greenhouse gas accounting metrics indicated that both marshes had a positive (warming) radiative balance, with the brackish marsh having a substantially greater warming effect than the freshwater marsh. That net respiratory emissions of CO2 and CH4 as estimated through chamber techniques were 2–4 times different from emissions estimated through EC requires additional understanding of the artifacts created by different spatial and temporal sampling footprints between techniques.

Nutria is an exotic, aquatic rodent that was introduced to Louisiana wetlands during the early 1930s and can make coastal restoration more challenging. From 1960 to 1990, greater than 36-million nutria were taken when the fur market was lucrative. By 2000, the fur market had collapsed and nutria populations increased. Nutria reach sexual maturity at 4–8 months, may have 2–3 litters per year, and average 13.1 young per female per year. Nutria generally have a small home range in marsh habitats occurring primarily in fresh and intermediate marsh. The organic nature of these soils makes them particularly susceptible to destruction with grazing. Nutria are opportunistic feeders, with a broad diet comprising more than 60 plant species in Louisiana and they are attracted to wetlands that contain a reliable source of nutrient-rich fresh water, such as river diversions and assimilation wetlands. It is imperative that restoration projects that increase input of nutrient-rich fresh water into wetlands have a nutria control program in place. Nutria can consume large amounts of marsh biomass and in certain cases can cause the collapse of marsh locally. Scientific studies investigating effects of nutria on marsh habitats consistently conclude that nutria grazing is damaging to marsh and young forest vegetation. It is generally accepted that nutria damage – in addition to larger scale subsidence, sea level rise, and salinity intrusion – can create an accelerated deterioration of wetlands. Nutria grazing on baldcypress and water tupelo seedlings is extensive and remains a major factor in the inability of baldcypress-water tupelo forests to regenerate. Projects designed to restore coastal swamp forests should include a nutria control component and suitable protection of transplants should be used to minimize mortality from grazing. Eruptions of populations of nutria can cause severe wetland damage and loss. Some areas of the coast have persistent populations creating “Hot Spots” of severe damage, especially in freshwater-intermediate salinity areas of Terrebonne, Barataria, and Breton Sound basins. Nutria densities are relatively low in the Chenier Plain currently compared to historic observations and harvest records. The Coastwide Nutria Control Program (CNCP) was implemented in 2002–2003 by the LDWF, and since then there has been a reduction in 70,000 acres of marsh damage from 2003 to 2010. Approximately 446,000 nutria were harvested in 2010 in the CNCP, primarily in the deltaic plain. When considering the costs of creating new wetlands (approximately $50,000–$70,000 per acre), the CNCP is a successful wetland conservation program that has produced measureable reduction in marsh damage. Since 2002–2003, 2,571,480 nutria have been harvested. This program is a success and, from a resource management perspective, should be continued with improvement and expansion if possible. Nutrient enrichment of coastal landscapes may cause nutria population growth and habitat damage. Coastal restoration projects with areas receiving nutrient enrichment should include nutria control to ensure plant productivity, establishment, and expansion.

We present here an integrated analysis of coastal deltaic floodplains in the active Atchafalaya Coastal Basin coupled to downstream deltaic estuaries to review how ecosystem properties self-organize around fluvial processes during river re-occupation as part of the delta cycle. The flood pulse of the river is critical to providing autogenic feedbacks between flow patterns, sediment delivery, vegetation productivity, and organic/inorganic accretion that produce spatial patterns of land elevation, habitat diversity, and estuary dynamics. Coastal deltaic floodplains form in the proximal region of an active delta as bar-shaped islands with interdistributary bays shape hydrogeomorphic zones influenced by both geophysical and ecological processes. Hydrogeomorphic zones in coastal deltaic floodplains of the proximal sedimentation zone can also be defined by time since subaerial emergence accounts for variability in vegetation community composition and soil successional development. The reduction in sedimentation and increase in both above- and belowground biomass associated with formation of hydrogeomorphic zones results in significant increase in organic matter density in soils, with higher N:P ratios reflecting the biotic feedback of ecological succession on delta floodplain development. In both the proximal and distal sedimentation regions, episodic events, such as river floods and cold fronts, control seasonal water levels, marsh platform inundation, and increase in elevation capital. In coastal deltaic floodplains, an increase in vegetation height and density has a twofold effect: it favors trapping of sediment on the islands; whereas an increase in roughness deflects water flow and sediment into the channels thus bypassing the marsh surface. There is evidence that this is in contrast to more constant positive feedback of vegetation on sedimentation in distal estuarine marsh platforms. Delta estuaries go through a transformation from a near-riverine estuary in the winter-spring season to a near-marine lagoon in the summer-fall season. Geomorphological displacement of vegetation types occurs as platform elevation increases in the proximal sedimentation zone as delta landform emerges, with specific vegetation dominating the respective subtidal, intertidal and supratidal hydrogeomorphic zones. This does not occur in the distal sedimentation zone that lack sediment input as marsh platform elevation decreases. This is due to presence of salinity and H2S that limit the capacity of biotic feedbacks to contribute to marsh stability. The growth of a coastal deltaic floodplain in the proximal sedimentation region of Atchafalaya Coastal Basin along with stable estuarine marshes in distal sedimentation region demonstrate the value of long-term riverine influence by preventing loss of wetland platform elevation.